1. Field of the Disclosure
The present disclosure is directed towards the surveying of wellbore orientation, including correcting magnetometer measurements for the effect of magnetization of the drill collar.
2. Background of the Disclosure
Surveying of boreholes is commonly performed by the use of instruments containing sets of three orthogonal accelerometers and magnetometers, which are inserted within the drillstring and used to measure the orientations of the local gravitational and magnetic field vectors. In order to measure the earth's magnetic field, which is used as a north reference from which wellbore azimuth may be computed, the instrument is placed within a section of a non-magnetic material extending between upper and lower ferromagnetic drillstring sections. These ferromagnetic portions of the drillstring tend to acquire remanent magnetization as they are repeatedly strained in the earth's magnetic field during transportation and drilling operations. The nominally non-magnetic portion of the drillstring may also acquire some lesser remanent magnetization as a result of imperfections. The result is that magnetometer measurements made by an instrument within a drillstring may measure not the undisturbed magnetic field, but the vector sum of the earth's field and an error field caused by remanent drillstring magnetization. Since the tool is fixed with respect to the drillstring, the error field is fixed with respect to the tool's coordinate system and it appears as bias errors on the magnetometer measurements, which can lead to errors in the determination of wellbore azimuth and trajectory unless measures are taken to compensate for these bias errors. Drilling fluid may contain magnetic particles such as steel filings from casing. The presence of such particles will impart magnetic permeability to the fluid, such that it attenuates the cross-axial field seen by the magnetometers. Such attenuation appears to be a scale factor error on the transversely sensitive magnetometers. In addition, ferromagnetic drill collars will develop an induced axial flux proportional to the axial component of the earth's field. At the ends of the ferromagnetic collars, the flux produces magnetic poles which affect the axially sensitive magnetometer. Since the effect is proportional to the axial component of the earth's field, it appears to be a scale factor error on the axially sensitive magnetometer. These apparent scale factor errors can also lead to errors in the determination of wellbore azimuth and trajectory unless compensatory measures are taken.
Estimation of magnetometer bias is often done for surveys at single stations. The single-station corrections normally assume that the error field is parallel to the drillstring; therefore sufficient information is available if the total magnetic field and/or dip angle can be provided from an independent source, such as a global geomagnetic model or a local reference measurement. Single-station corrections of this type suffer from inaccuracy in certain attitudes, and they do not normally correct for magnetic disturbances which are not parallel to the drillstring. More recently, multi-station magnetic corrections have been developed. These techniques may increase the range of attitudes in which corrections can be applied, and they can also identify some off-axis magnetic anomalies, while providing improved quality control. An axiom of multi-station corrections is the assumption that magnetic disturbances are unchanged over each group of survey stations being processed. See, for example, U.S. Pat. No. 6,179,067 to Brooks having the same assignee as the present disclosure, which is incorporated herein by reference in its entirety.
While the assumption that magnetic disturbances are unchanged over a group of survey stations is not unreasonable in the case of remanent collar magnetization or attenuation of the cross-axial field by drilling mud, it is known that the effect of the induced component of drill collar magnetization will change depending on the angle between the collar and the earth's field. In order to correct for induced magnetization accurately, it must therefore be modeled as an axial scale factor error rather than as a bias. However, the solution of the multi-station problem tends to be very poorly conditioned if a solution is sought for both an axial bias and an axial scale factor, because these two parameters are closely correlated. Therefore it has become common practice to ignore the axial scale factor error and to solve multi-station problems for axial bias only. Such multi-station solutions are inaccurate, because they fail to account for the variation in axial error field between stations which results from induced magnetization or axial scale factor error.